Ivanovic said she was not bothered by pain from a recent injury to her right thumb, attributing her poor performance more to lack of match practice.

The 20-year-old Serbian withdrew from the Olympics because of the injury and has played little since her third-round exit at Wimbledon, struggling to retain the form that won her the French Open in June.

"I really struggled and made too many unforced errors and my serve was not working really well," said Ivanovic. "Obviously, it's very frustrating because I know I can play so much better."

Asked if she had been feeling the pressure of being the top ranked player, she replied: "No, I think I deserve that position. I won a Grand Slam and I was playing really well.

"Obviously if you would ask me at the moment if I'm playing like number one, probably not, but I can't judge too much on that because I really haven't had [a] chance to practice.

"This kind of loss I had today is just incentive to work harder, to go back on the court and to keep working hard and practising and improving."

Ivanovic, who flew to Australia before coming to New York to get treatment on her thumb, said she has totally recovered from the injury.

"Today I didn't feel it, and also [the] last match was good. So I'm just happy to be back on track without pain. Now all I have to do is put some hard yards on the court and go back out there and work hard."

Ivanovic's defeat was the earliest exit by a women's top seed at the US Open since tennis turned professional in 1968.

The top players in the WTA ranking are so close on points that Ivanovic could still keep the top spot after her early exit, depending on how the other players do in the tournament. The four women with a chance of ousting her are Jelena Jankovic, Serena Williams, Dinara Safina and Elena Dementieva.